Phosphorous Acid %

Phosphorous Acid, H₃PO₃, is a diprotic acid that is a commonly intermediate material used in the creation other phosphoric compounds, phosphonates, and phosphites.
Phosphorous acid is also used in some pesticide, water treatment, plastics (stabilizer and fire retardant), and more.
Its phosphonates are can often be found in various chelating agents. Some notable phosphonates include: cyclic phosphonate, glyphosate, and several metal chelants, for example EDTA.

Posphorous Acid as a fire retardant:
Phosphorous Acid, and particularly cyclic phosphonate, is a fire retardant that, upon an endothermic reaction, converts to phosphoric acid. The phosphoric acid absorbs water from the burning plastic and produces char; this char layer prevents the further consumption of the material.

Additional Notes:
* When a material with phosphorous acid as a fire retardant is burned a glassy char will form.
* Upon heating around 200°C (392°F), phosphorous acid will convert to phosphoric acid and phosphine.